Organic Chemistry

Section 23.14 Tertiary Structure of Proteins 991

Coil Conformation

Generally, less than half of a globular protein is in an or sheet

(Figure 23.10). Most of the rest of the protein is still highly ordered but is difficult to

describe. These polypeptide fragments are said to be in a coil conformationor a loop

conformation.

a-helix b-pleated

>Figure 23.10

The backbone structure of

carboxypeptidase A:

segments are purple;

sheets are indicated

by flat green arrows pointing in

the direction.N¡C

b-pleated

a-helical

23.14 Tertiary Structure of Proteins

The tertiary structureof a protein is the three-dimensional arrangement of all the

atoms in the protein. Proteins fold spontaneously in solution in order to maximize their

stability. Every time there is a stabilizing interaction between two atoms, free energy is

released. The more free energy released (the more negative the ), the more stable

the protein. So a protein tends to fold in a way that maximizes the number of stabiliz-

ing interactions (Figure 23.11).

¢G°

AN ATTEMPT

TO IMPROVE ON NATURE

Chemists are currently studying

which are polymers of acids. These peptides have

backbones one carbon longer than the peptides nature synthe-

sizes using acids. Therefore, each acid

residue has two carbons to which side chains can be attached.

Like fold into relatively sta-

ble helical and pleated sheet conformations, causing scientists

to wonder whether biological activity might be possible with

such peptides. Recently, a with biological activity

has been synthesized—one that mimics the activity of the hor-

b-peptide

a-polypeptides,b-polypeptides

a-amino b-amino

b-amino

b-peptides,

B-PEPTIDES: mone somatostatin. There is hope that will pro-

vide a source of new drugs and catalysts. Surprisingly, the pep-

tide bonds in are resistant to the enzymes that

catalyze the hydrolysis of peptide bonds in

This resistance to hydrolysis means that a drug

will have a longer duration of action in the bloodstream.

H 3 N

R

O−

O

CH C

-amino acid

+

H 3 N

R R′

O−

O

CH CH C

-amino acid

+

b-polypeptide

a-polypeptides.

b-polypeptides

b-polypeptides

PROBLEM 34

How long is an that contains 74 amino acids? Compare the length of this

with the length of a fully extended peptide chain containing the same number of amino

acids. (The distance between consecutive amino acids in a fully extended chain is 3.5Å.)

a-helix a-helix

BRUI23-959-998r2 29-03-2003 1:37 PM Page 991